The hydrometeorological disaster that affected the south of Valencia on October 29, 2024, leaving behind nearly 230 deaths, thousands of homeless, 200,000 unusable vehicles and thousands of tons of putrid mud, is a Mediterranean-type Cevennes event, particularly intense due to climate change.
[ DANA = Depresión Aislada en Niveles Altos ou dépression isolée à haute altitude]
Its ingredients are a stationary cold drop (isolation of cold polar air at altitude, quasi-stationary in geographical position and therefore minimum pressure at altitude), a very high seawater temperature and a sea wind that corners a humidity-laden atmosphere on a mountain foothill. The intense rain, over a very long period, induces a gigantic wave of water and mud that breaks below.
[See FAQ "Vortex polaire et vagues de froid"]
The ingredients of the "dana"
The cold drop
The "jetstream" or polar jet stream is a west-to-east flow that is maximum in the vicinity of the tropopause (around 10 or 15 km altitude in general very useful for long-haul flights to the east), and which separates the polar atmosphere from that of the mid-latitudes. This current is sometimes subject to undulations that cut off from the large-scale flow, migrate to lower latitudes and become stationary, maintaining areas of high instability: this is the phenomenon of the "cold drop". In coastal areas, the cold drop is fed in the lower layers by a flow of warm and humid air from a very warm Mediterranean. The whole produces intense rainfall for a long time. The atmospheric situation at the level of 500 mbar (~ 5500m) the day after the disaster is illustrated in the figure below:
Sea surface temperature
The sea surface temperature determines its capacity to evaporate and therefore to supply the atmosphere with water vapor, but on 29 October it reached 23ºC, a particularly high level for this season.
Sea wind and mountain foothills
The sea wind carries inland an atmosphere very charged with water vapor sucked in by the cold drop. This atmosphere, in contact with the mountain foothill, is pushed upwards and therefore cools. This leads to condensation of water vapor and intense precipitation localized on the relief. Precipitation occurs over a very long period of time as the atmosphere continues to be supplied with water vapor from the Mediterranean. The quantities of water involved cannot be channeled through the water network and surge downstream, carrying the soil and vegetation with them.
Area affected by the disaster
This is the south of Valencia and more precisely the south of the Turia River, which was diverted after the previous disaster in 1957. It encompasses the Albufera and the plain that extends to the mountain foothills to the west and more occasionally the wine-growing areas on the relief. It should be noted that the most affected areas did not have rain at the height of the disaster! The most affected town is Paiporta which was submerged by a 3m wave of mud.
Sequences of events On Thursday 23
The Spanish meteorological agency (AEMET) issued a warning (cold drop) for the Valencia region; On the evening of Monday 28 the AEMET clarified its warning about possible extreme events for the surroundings of Valencia; On the morning of Tuesday 29 the AEMET specified that the "cold drop" warning concerned the immediate vicinity of Valencia: the University of Valencia closed and some of the cities concerned closed their schools, the regional authority (Generalidad), which had previously abolished the disaster prevention service, did not take this into account and did not trigger the activation of measures to ensure the safety and protection of people and property; The agency in charge of hydraulics issued a warning to the president of the Generalidad who considered that the situation did not require intervention insofar as the retention basins were empty! Various signs are worrying, including the saturation since midnight of the "112" and the appearance at 2:30 p.m. of a flood of plants (see figure below) in a watercourse that had been dry until then.
The president of the Generalidad, who is responsible for issuing an alert, has lunch with a journalist until late in the afternoon despite multiple calls; The bulk of the disaster took place between 5:30 p.m. and 8 p.m., including the famous 3m wave in Paiporta, while in the absence of rain, no worrying signs were perceived by the population; The general alert was finally launched on mobile phones at 8:10 p.m.!
Conclusions
- Nearly 230 dead, many of them in low houses whose exits were blocked by vehicles carried by the wave;
- Massive destruction of buildings; An economy bled dry, including rice cultivation (chemical and bacterial pollution of the whole of the Albufera and the rice fields that border it), citrus groves, viticulture, industrial and commercial enterprises;
- A very rapid recovery for some localities (Alfafar...) much more problematic and over a long period of time for others (Paiporta...);
- But also, an exceptional solidarity from all of Spain and elsewhere;
- The diversion of the Turia River prevented the city of Valencia from being in the perimeter of the disaster... It remains to ensure that the localities on the southern periphery benefit from the same protection...
The Generalidad's behavior is all the more unacceptable given that it has for many days refused the aid that was offered to it at the national level but could not be imposed on it!